signal fire vs top light

signal fire

noun
  • A fire lit to convey a message or serve as a beacon, by either light or smoke. 

top light

noun
  • Light from above; toplighting. 

  • Any glazing above a main window, partition or door 

  • A skylight. 

  • The upper (and usually smaller) part (casement) of a window, often hinged a the top 

How often have the words signal fire and top light occurred in a corpus of books? (source: Google Ngram Viewer )